1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a hollow rack for use as a steering rack in a steering system of a vehicle, for example, an automobile.
2. Description of the Related Art
Conventionally, there has been known a method and an apparatus for forming a rack corresponding to the tooth profile of a die at a flat tooth profile processing portion of a pipe material by inserting or removing a long rod-like metal core having a plurality of expanded diameter sections into/from the pipe material held by dies composed of an upper die and a lower die through openings at both ends alternately (for example, patent document 1: paragraphs [0023] to [0027] and FIG. 4 of Jpn. Pat. Appln. KOKAI Publication No. 2002-86243).
Further, there has been known a method and an apparatus for, by using a metal core push rod to be inserted/removed into/from an iron pipe held by a die assembly composed of an upper die and a lower die alternately through openings on both ends, forming the rack corresponding to the tooth profile of the die in an iron pipe by pressing the metal core much shorter than the push rod into the iron pipe (for example, patent document 2: paragraphs [0022] to [0037] and FIGS. 1 to 9 of Jpn. Pat. Appln. KOKAI Publication No. 2006-26703).
More specifically, according to the patent document 2, an iron pipe having a flat tooth profile processed portion compressed into a semi-crescent shape is held between an upper die having the tooth profile portion and a lower die. Next, each of plural metal cores supported by metal core accommodation portions disposed on both sides of the die assembly is pressed into the iron pipe successively by the metal core push rods which are inserted/removed into/from the iron pipe alternately from both sides of the die assembly.
In pressing in, the metal core push rod inserted into the iron pipe presses a single metal core pulled out of the metal core accommodation portion up to a state in which it passes through the tooth profile of the upper die completely. Next, at the same time when one of the metal core push rods is retreated, the other metal core push rod pulls out other metal core from the other metal core accommodation portion and presses it into the iron pipe. The metal core pulled out of the metal core accommodation portion on one side by the metal core push rod and pressed into the iron pipe, is pushed back by the other metal core push rod through the other metal core and returned to the metal core accommodation portion on the one side. When the metal cores are pressed into the iron pipe alternately, a projection provided at the front end of the metal core push rod is engaged with an oval depression provided in an end face of the metal core pressed by the metal core push rod so as to stop rotation of the metal core.
By pressing the metal core into the iron pipe from the right and left side alternately, the fabric of a portion to be processed into the tooth profile of the iron pipe is fluidized plastically toward a tooth profile outside from inside of the iron pipe so as to form a rack corresponding to the tooth profile in the iron pipe.
The metal core accommodation portion and the tooth profile of the die assembly are discontinuous and the metal core accommodated in the metal core accommodation portion is supported by a spring so that it is not moved from the accommodation position unexpectedly due to vibration or the like. Thus, although the metal core and the metal core push rod are stopped from rotating by engagement between the oval depression and the projection, there is a fear that the metal core may slip out of the metal core push rod due to a force exerted when it is pushed out of the metal core accommodation portion by the metal core push rod. If the metal core is pressed by the metal core push rod with stoppage of rotation of the metal core released, the metal core might rotate freely around its axis. As a result, the metal core is pressed into the iron pipe with an appropriate posture of the metal core with respect to the portion to be processed into the tooth profile of the iron pipe, not only does a processing failure occur, but also an excessive load is generated at that time, thereby possibly damaging the manufacturing apparatus.
According to the technology of the patent document 2, the other metal core already pressed in is pushed back by the metal core to be pressed in contact with the metal core push rod inserted into the iron pipe. However, the metal cores are not formed into a structure preventing them from rotating with respect to each other. Additionally, the other metal core to be pushed back is pressed into a position where it has passed the tooth profile portion completely. Thus, the other metal core to be pushed back rotates freely around its axis thereby likely an appropriate positional relationship with the tooth profile processed portion of the iron pipe being degraded. Then, the tooth profile processed portion of the iron pipe is restored to some extent after fluidized plastically and the other metal core is pushed back through that portion. Thus, not only does a processing failure occur but also there is a possibility that an excessive load may be generated thereby damaging the manufacturing apparatus.
According to the technology of the patent document 1 using the long rod-like metal core, when the long metal core pushed into the pipe member is pulled back, the metal core can be broken due to a load applied by the tooth profile processed portion sprung back.
Further, to insert/remove the long rod-like metal core into/from the pipe material alternately, a driving portion having a capacity which applies a pressure for inducing the plastic fluidity is needed for each long rod-like metal core and these driving portions are disposed on both sides of the die assembly. Usually, a hydraulic cylinder is used in a pair of the driving portions. Thus, the manufacturing apparatus is of large scale.
In a pair of the hydraulic cylinders which constitute the driving portion, a long rod-like metal core is connected to their cylinder rods and the metal core is moved in a direction of extension of its axis. Further, the hydraulic cylinder on one side needs to be disposed with an interval longer than the length of the pipe material secured with respect to the die assembly. By considering a moving distance of the cylinder rod of each of the pair of the hydraulic cylinders, installation space for the manufacturing apparatus is determined. Thus, the manufacturing apparatus described in the patent document 1 is disadvantageous in its large scale and its large installation space.
Contrary to this, according to the technology of the patent document 2, the possibility that the metal core may be broken is low because it is much shorter.
However, the manufacturing apparatus described in the patent document 2 requires a pair of driving portions constituted of hydraulic cylinder on both sides of the die assembly in order to reciprocate the pair of the metal core push rods for pressing in the metal core from the right and left sides of the pipe material alternately. Thus, the manufacturing apparatus is of large scale.
The manufacturing apparatus described in the patent document 2 is advantageous for reducing the installation space as compared with the manufacturing apparatus described in the patent document 1. However, because a pair of the driving portions constituted of a hydraulic cylinder for reciprocating the metal core push rod are disposed on both sides of the die assembly, there is a room for improvement in reduction of the apparatus size.
According to the technology described in the patent document 2, a rack corresponding to the tooth profile of the die assembly can be formed by fluidizing the fabric of the tooth profile processed portion of the pipe material plastically outward from inside of the pipe material. According to such a manufacturing method, the tooth profile processed portion is inevitably attached to the tooth die and thus, the processed pipe material needs to be separated from the tooth profile and taken out of the die assembly.
However, the patent document 2 describes nothing about the technology of removing the pipe material attached to the upper die of the die assembly.
To separate a formed product attached to the upper die of the die assembly in various pressing units, a technology for building a knock out unit in the die assembly has been known.
This knock out unit is constituted of a plurality of knock out bars provided on the die assembly and driving means such as a hydraulic cylinder which pushes or pulls these bars with respect to the surface of the upper die. By building such a knock out unit in the upper die of the die assembly of the hollow rack manufacturing apparatus, the work of separating the pipe material attached to the upper die from the upper die can be automated.
However, building the knock out unit into the die assembly inevitably complicates the structure of the die assembly and accompanied by this, die assembly cost is increased and maintenance of the die assembly is more troublesome.